Method of generating combined look-up table, and image processing apparatus and image forming apparatus

ABSTRACT

An information processing apparatus generates a combined LUT that performs color matching and color separation in an integrated manner, based on an LUT for color matching and an LUT for color separation. Input points of the combined LUT are set such that output corresponding to the input points of the combined LUT corresponds to input points of the LUT for color separation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology which generates a singlelook-up table by combining a look-up table for color matching and alook-up table for color separation that are used by an image processingapparatus.

2. Description of the Related Art

When forming an image on a printing medium using an image formingapparatus, if color component data representing the image to be printedand color component data that can be interpreted by the image formingapparatus are different, it is necessary to convert the color componentdata. Converting the color component data includes, for example,performing color matching processing, converting a color space thatrepresents the image (for example, converting from a RGB color space toa CMYK color space; referred to as “color separation processing”), gammacorrection, and converting the number of tones of the color componentdata. In some cases deterioration in image quality occurs due to variousfactors when converting the color component data.

Japanese Patent Laid-Open No. 9-294212 discloses technology that reducesdeterioration in image quality caused by a quantization error thatoccurs when reducing the number of tones of image data using an LUT(look-up table). According to the technology disclosed in JapanesePatent Laid-Open No. 9-294212, an LUT is provided such that intervalsbetween lattice points become smaller if the density is lowered for acolor space of color component data that represents image data.

Color matching processing and color separation processing can beperformed using respectively different LUTs. In contrast, by using asingle LUT (hereunder, referred to as “combined LUT”) in which an LUTfor color matching and an LUT for color separation are combined, colormatching processing and color separation processing may be performed atthe same time.

A conventional combined LUT will now be described by referring to FIG. 9to FIG. 13.

FIG. 9 is a view that conceptually illustrates an LUT for color matchingprocessing. In FIG. 9, each lattice point indicates a sample point(input point) of the LUT. FIG. 10 is a conceptual diagram when FIG. 9 isviewed from a gray axis (axis where R=G=B). At the time of colormatching processing for image formation, in general the area of L* inthe L*a*b* color system is compressed. Therefore, in FIG. 10, thegradient decreases in a dark portion and in a light portion (a portionin which an input value is small and a portion in which an input valueis large).

FIG. 11 is a view that conceptually shows an LUT for color separationprocessing. FIG. 12 is a conceptual diagram of FIG. 11 when viewed fromthe gray axis. Although the relation between input values and outputvalues is originally as shown by the solid line (curve) in FIG. 12, foran input value that is other than a sample point, a linearlyinterpolated value as shown by the dashed line in FIG. 12 is output.Therefore, as shown in FIG. 12, sample points are set at intervals thatgrow smaller as the area of the gradient of curve increases, so that aninterpolation error is smaller.

FIG. 13 is a view that conceptually illustrates an interpolation errorcaused by an interpolation interval of a 3D-LUT after combinationaccording to the conventional method. FIG. 13 illustrates a case inwhich points obtained by inputting output values (32, 128, 211) obtainedfrom interpolation sample points of a color matching table into a colorseparation table are linearly interpolated as sample points. In thefigure, it is found that an interpolation error with the original colorseparation table is large between the input values 32-128 in particular.Because of this error, there is a problem that, in color separationprocessing, adjustment of a slight ink amount of a gray component isthrown out of balance.

As described above, when performing color matching processing and colorseparation processing using a conventional combined LUT, there is aproblem that there are large interpolation errors and the image qualityof the image to be formed deteriorates.

Further, although the technology disclosed in Japanese Patent Laid-OpenNo. 9-294212 takes into consideration decreasing interpolation errorswhen using a single LUT, it does not take into consideration generatinga combined LUT with small interpolation errors.

Furthermore, as a method for solving the problem whereby interpolationerrors are generated as described using FIG. 13, although errors can bemade smaller by increasing the number of sample points of a combined LUTand a deterioration in image quality can thereby be reduced, the dataamount of the combined LUT increases. An increase in the data amount ofthe combined LUT becomes a particularly serious problem, for example, ina case in which an image forming apparatus with a limited memorycapacity performs color matching processing and color separationprocessing.

The present invention was made in view of the above circumstances. Thatis, a feature of the present invention is to provide technology thatreduces a deterioration in image quality of an image to be formed whilesuppressing an increase in a data amount of a combined LUT whengenerating a single combined LUT by combining an LUT for color matchingand an LUT for color separation.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided amethod of generating a combined look-up table for converting a firstcolor component data into a third color component data by combining afirst look-up table for converting by color matching the first colorcomponent data that is generated and input by a first apparatus, andrepresented in a first color space into a second color component datathat is represented in the first color space taking into considerationcolor reproduction characteristics of a second apparatus, and a secondlook-up table for converting by color separation the second colorcomponent data into the third color component data that is representedin a second color space used by the second apparatus, comprises a stepof generating a combined look-up table in which color component dataafter color separation that corresponds to each input point of thesecond look-up table is made to correspond with corresponding colorcomponent data before color matching in a case in which each piece ofcolor component data corresponding to each input point of the secondlook-up table is taken as color component data obtained using the firstlook-up table.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a print system according to theembodiments;

FIG. 2 is a view illustrating one example of the configuration ofsoftware that is executed on a PC according to the embodiments;

FIG. 3 is a view illustrating the details of image processing(conversion of color component data) performed by a printer driver of aPC according to the embodiments;

FIG. 4 is a view illustrating the details of processing performed by acorrection processing unit (for a printer) of the printer driver;

FIG. 5 is a view that schematically shows a difference between a colorreproduction space of a printer according to the embodiments and a colorreproduction space of a display device according to the embodiments;

FIG. 6 is a view that conceptually illustrates gamut compression towardsthe saturation (S) direction on an SV plane;

FIG. 7 is a conceptual diagram of calculation processing at step S1403of FIG. 14;

FIG. 8 is a conceptual diagram of a combined LUT according to theembodiments;

FIG. 9 is a view that conceptually shows an LUT for color matchingprocessing;

FIG. 10 is a conceptual diagram showing FIG. 9 when viewed from the grayaxis (axis of R=G=B);

FIG. 11 is a view that conceptually shows an LUT for color separationprocessing;

FIG. 12 is a conceptual diagram of FIG. 11 when viewed from the grayaxis;

FIG. 13 is a conceptual diagram of a conventional combined LUT whenviewed from the gray axis; and

FIG. 14 is a flowchart showing the flow of processing which generates acombined LUT from a matching table and a color separation table of acorrection processing unit for a printer.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to attached drawings. Each embodiment described belowwill be helpful in understanding a variety of concepts from the genericto the more specific.

It should be noted that the technical scope of the present invention isdefined by claims, and is not limited by each embodiment describedbelow. In addition, not all combinations of the features described inthe embodiments are necessarily required for realizing the presentinvention.

First Embodiment

According to the present embodiment, a case is described in which animage is printed using the print system 110 shown in FIG. 1.

A personal computer (PC) 100 is one example of an image processingapparatus that converts color component data. A CPU 101 controls eachcomponent of the PC 100 by executing an operating system (OS),application programs, device drivers, and the like. A RAM 102 is amemory that functions as a work area of the CPU 101. A ROM 103 is amemory that stores startup programs etc. such as a BIOS and the like.

A hard disk drive (HDD) 104 is a storage device that stores computerprograms such as an application program, an OS, and various devicedrivers, as well as a test pattern image for a printer or image data tobe printed, and the like. A communication I/F 105 is an interface suchas a USB, an IEEE 1394, a wired LAN, or a wireless LAN. Thecommunication I/F 105 performs data communication with an externaldevice such as a printer 108. A display I/F 106 is an interface forsending image information or the like to a display device 109 that isconnected to (or integrated with) the PC 100.

An operation unit 107 includes a pointing device or a key input deviceand the like. The PC 100 receives instructions from a user through theoperation unit 107.

The printer 108 is an image forming apparatus that forms an image on aprinting medium by an inkjet method or a laser beam method. Although inthis case the printer 108 is taken as an apparatus that uses binarizedCMYK data for forming an image, the present invention is not limitedthereto. For example, the printer 108 may use binarized CMY data (colorcomponent data for CMY color spaces) or may use multi-level colorcomponent data.

The display device 109 is a liquid crystal display or the like fordisplaying an image or the like.

In FIG. 1, although the PC 100 and the printer 108 are shown asrespectively independent devices, a configuration may be adopted inwhich these two devices are integrated. For example, the printer 108 maycomprise the same image processing function as the PC 100 and mayconvert color component data. Thus, for example, a printer that canperform so-called “direct printing” by connecting a digital camera canalso perform color matching processing and color separation processingusing the combined LUT (look-up table) according to the presentembodiment.

FIG. 2 is a view that shows one example of the configuration of softwarethat is executed at the PC 100.

In FIG. 2, an OS 201 is an operating system that controls the PC 100.Applications 202 are, for example, image editing software and documentcreation software and the like. An operation unit driver 207 is a devicedriver for controlling the operation unit 107. A printer driver 208 is adevice driver for controlling the printer 108. A display device driver209 is a device driver for controlling the display device 109.

Hereunder, the processing that is conducted when the PC 100 instructsthe printer 108 to print an image is briefly described by referring toFIG. 2.

For example, an application 202 such as image editing software makes aprint request to the OS 201. Upon receiving the print request from theapplication 202, the OS 201 issues a print instruction to the printerdriver 208 corresponding to the printer that will execute printing. Atthis time, for example, image data that is represented by RGB data issent from the application 202 to the printer driver 208 through the OS201.

The printer driver 208 converts the RGB data into data that can beinterpreted by the printer 108 (for example, binarized CMYK data) inaccordance with the print instruction that is received from the OS 201,and outputs the converted data to the printer 108. More specifically,for example, when the printer 108 is a raster printer, the printerdriver 208 sequentially performs image correction processes for theimage data in accordance with the print instruction from the OS 201. Theprinter driver 208 then, for example, sequentially rasterizes thecorrected image data on an RGB 24-bit page memory (the number of bits isone example, and is not limited to 24 bits). After rasterization of theimage data is completed, the printer driver 208 converts the contents ofthe RGB 24-bit page memory into a data format capable of beinginterpreted by the printer 108 (for example, binarized CMYK data) andoutputs the data to the printer 108.

FIG. 3 is a view illustrating the details of image processing(conversion of color component data) performed by the printer driver208.

An image correction processing unit 301 performs correction processingfor the image data received from the OS 201. More specifically, forexample, the image correction processing unit 301 converts R, G and Bcolor information of the image data into brightness and color differencesignals, performs exposure correction processing for the brightnesssignal, and then performs inverse conversion of the corrected brightnessand color difference signals into R, G and B color information. Eachpiece of RGB 8-bit image data that the image correction processing unit301 outputs corresponds to a color to be reproduced on the displaydevice 109. For example, for coordinate values of a CIE L*a*b* colorsystem that is a uniform color space, the image data represents thecolors (L_Monitor, a_Monitor, b_Monitor).

A correction processing unit 302 for a printer rasterizes the image dataand expands the raster image on a RGB 24-bit page memory. Further, thecorrection processing unit 302 for a printer performs color matchingprocessing that executes color reproduction space mapping, colorseparation processing into C, M, Y and K, gamma correction processing,halftone processing, thus generating C, M, Y and K data that conformswith the color reproducibility of the printer for each pixel of theimage data, and outputs the C, M, Y and K data to the printer 108.

FIG. 4 is a view illustrating the details of processing performed by thecorrection processing unit 302 for a printer. Hereunder, to simplify thedescription, a color space that is normally represented in threedimensions is schematically represented in two dimensions (see FIG. 5and the like).

In FIG. 4, although a color matching processing unit 402 and a colorseparation processing unit 403 are displayed separately for the purposeof description, the correction processing unit 302 for a printeraccording to the present embodiment performs color matching processingand color separation processing using a combined LUT. Therefore, asdescribed in detail later, the color matching processing unit 402 andthe color separation processing unit 403 are integrated. Further, anoutput gamma correction processing unit 404 may also be integrated withthe color matching processing unit 402 and the color separationprocessing unit 403.

An image signal input unit 401 receives image data that has beencorrected by the image correction processing unit 301, and passes thedata to the color matching processing unit 402.

The color matching processing unit 402 adjusts the consistency of thecolor reproduction space of the printer 108 and the color reproductionspace of the display device 109. More specifically, by executingcompression for an unreproducible region with the printer 108, the colorreproduction space (monitor gamut) of the display device 109 isassociated with points inside the color reproduction space (printergamut) of the printer 108. Thus, each piece of 8-bit RGB data isconverted into pieces of 8-bit R′G′B′ data that corresponds to points onthe printer color reproduction space. This is described specificallyhereunder by referring to FIG. 5 and FIG. 6.

FIG. 5 is a view that schematically shows a difference between a colorreproduction space of the printer 108 and a color reproduction space ofthe display device 109. As described above, the image data correspondsto colors to be reproduced on the display device 109 and, for example,represents colors (L_Monitor, a_Monitor, b_Monitor) for the L*a*b* colorsystem.

However, as will be understood from FIG. 5, the color reproduction spaceof the display device 109 and the color reproduction space of theprinter 108 do not match on a uniform color space such as, for example,the L*a*b* color system. Therefore, a region that is on the inner sideof the color reproduction space of the display device but is outside thecolor reproduction space of the printer as shown in FIG. 5(diagonal-line portion), does not properly undergo image formation atthe printer 108.

More specifically, a case may be considered in which, as describedlater, image data that is represented on the display device 109undergoes RGB-CMYK conversion at the color separation processing unit403, gamma correction processing at the output gamma correctionprocessing unit 404, and binarization processing at a halftoneprocessing unit 405. In this case also, there is a possibility that aregion for which image formation is not properly performed, i.e. aregion on the color space that cannot be represented at the printer 108(diagonal-line portion in FIG. 5) may occur in the image data that isoutput from the image output unit 406 to the printer 108.

Accordingly, it is necessary that the printer 108 performs imageformation with pseudo colors (L*a*b* values), i.e. pseudo colors thatdiffer from the coloring of the display device 109, with respect to aregion on the color space that is reproducible by the display device 109and is not reproducible by the printer 108 (diagonal-line portion ofFIG. 5).

Hereunder, as one example of color matching processing, a case isdescribed in which an L*a*b* space as a uniform color space is convertedinto HSV space as a cylindrical coordinate system, and compression ofthe color space is performed for the HSV space. In this connection,conversion to HSV space is performed using the conversion formulaH=atan(b/a), S=sqrt(a*2+b*2), V=L*. In this case, atan(x) is a functionfor calculating an arc tangent of x. Further, sqrt (x) is a function fordetermining √x.

The color matching processing unit 402 performs processing so that agamut (monitor gamut) produced by RGB at the display device 109 on anL*a*b* space enters inside a gamut (printer gamut) produced by RGB atthe printer 108. More specifically, for example, the color matchingprocessing unit 402 compresses the monitor gamut by performingprocessing that lowers the saturation S while maintaining a brightnessL* or the like.

FIG. 6 is a view that conceptually shows gamut compression towards thesaturation (S) direction on an SV plane. By this processing, a set ofL*a*b* values in the printer gamut are obtained that correspond to RGBvalues at the display device 109. More specifically, thepost-compression monitor gamut is made to fit into the printer gamut.For example, taking L*a*b* values as a key, sets of the monitor (R, G,B) and the printer (R′, G′, B′) are determined so that a colordifference ΔE (=sqrt((L*′-L*)²+(a*′-a*)²+(b*′-b*)²)) is minimized. It isthereby possible to obtain R′G′B′ values at the printer that correspondto RGB values at the display device 109.

The R′, G′, B′ values obtained in this manner are sent to the colorseparation processing unit 403 as the result of color matchingprocessing.

In this connection, the above described color matching processing canalso be implemented using an LUT. More specifically, the color matchingprocessing unit 402 converts RGB values of the input image data intoR′G′B′ values for the printer 108 by referring to the LUT. Since thesize of a printer gamut will differ according to the type of printingmedium (such as paper), according to the present embodiment an imageprocessing parameter setting unit 411 holds a plurality of matchingtables 407 as an LUT for each printing medium. The color matchingprocessing unit 402 selects a matching table 407 based on a paper type(kind of printing medium) that is obtained from a paper type input unit410.

Next, color separation processing is explained.

The color separation processing unit 403 converts RGB data (moreexactly, R′G′B′ data that is the output from the color matchingprocessing unit 402) into CMYK data.

As a conversion method for converting R′G′B′ data into CMYK data, forexample, a method that uses color masking is known. A color conversionmethod that uses color masking is shown in the following formulas.

D r = −log   (R/255) D g = −log   (G/255)D b = −log   (B/255) $\begin{pmatrix}C \\M \\Y\end{pmatrix} = {\begin{pmatrix}1 & m_{r} & y_{r} \\c_{g} & 1 & y_{g} \\c_{b} & m_{b} & 1\end{pmatrix}^{- 1}\begin{pmatrix}D_{r} \\D_{g} \\D_{b}\end{pmatrix}}$

By computing these formulas, the CMY values are obtained. In thisconnection, various methods are available as methods of determining asignal value of K (black). For example, it is possible to substitute avector [Dr−K Dg−K Db−K]t obtained by subtracting K values from therespective vector elements for the rightmost side of the abovedeterminant. Subsequently, by obtaining a masking matrix by trial anderror while restricting the K value by utilizing the condition that theCMY color signal values corresponding to the ink amount are alwayspositive or zero, the value of K can be determined.

According to the present embodiment, it is assumed that the colorseparation processing unit 403 performs color separation processingusing LUTs. Similarly to the color matching processing, the imageprocessing parameter setting unit 411 holds a color separation table 408for each printing medium in the form of an LUT.

CMYK data obtained by color separation processing is sent to the outputgamma correction processing unit 404.

The output gamma correction processing unit 404 performs gammacorrection processing in order to compensate for non-linearcharacteristics with respect to brightness of the printer 108, andconverts CMYK data to C′M′Y′K′ data. The output gamma correctionprocessing unit 404 refers to an LUT for each color component to performgamma correction processing. Similarly to the color matching processing,the image processing parameter setting unit 411 holds gamma correctiontables 409 for each printing medium in the form of an LUT.

The halftone processing unit 405 converts multiple value (for example, 8bits for each color) C′M′Y′K′ data into binary C″M″Y″K″ data that can beprinted by the printer 108 and performs halftone processing.

One method of performing this halftone processing involves ofrespectively applying, for example, a Bayer-type 16×16 matrix toC′M′Y′K′ images that are input. Subsequently, the halftone processing isrealized by assigning 1 when a pixel value on the corresponding image islarger than an element on the matrix, and assigning 0 when the pixelvalue is less than an element on the matrix. Another halftone processingmethod such as an error diffusion method can also be used.

Each piece of 1-bit C″M″Y″K″ data that the printer 108 is capable ofprinting that is obtained in this manner is sent to the printer 108 andformed as an image on a printing medium.

<Generation of Combined LUT>

Next, the method of generating a combined LUT from the matching table407 and the color separation table 408 is described. As described above,according to the present embodiment the color matching processing unit402 and the color separation processing unit 403 (see FIG. 4) areintegrated, and the correction processing unit 302 for a printer (seeFIG. 3) refers to the combined LUT to convert RGB data to CMYK data.Conversion from RGB data to CMYK data is one example of conversion and,for example, the combined LUT may convert RGB data into CMY data.

FIG. 14 is a flowchart showing the flow of processing which generates acombined LUT from the matching table 407 and the color separation table408. The processing that generates the combined LUT can be performed byan arbitrary information processing apparatus that can execute theprocessing of each step of FIG. 14 (for example, the PC 100 shown inFIG. 1). Hereunder, a case is described in which the PC 100 generatesthe combined LUT.

At step S1401, the PC 100 acquires the matching table 407 and the colorseparation table 408. The matching table 407 and the color separationtable 408 are, for example, sent to the PC 100 via the communication I/F105. Alternatively, the information processing apparatus may generatethe matching table 407 and the color separation table 408 in annot-shown step, the tables may then be stored in a storage device suchas the HDD 104, and acquired from the storage device.

At step S1402, the PC 100 acquires color component data (R′G′B′ data)corresponding to each input point of the color separation table 408.

At step S1403, taking each piece of color component data acquired atstep S1402 as the output at the matching table 407, the PC 100calculates the corresponding color component data (RGB data) prior tocolor matching. FIG. 7 is a conceptual diagram of the calculationprocessing at step S1403. When the concept illustrated in FIG. 7 isapplied in a 3D-LUT, computation of an input value (RGB) correspondingto a certain output value (RGB) is determined. More specifically, theoutput value (RGB) is compared with a value on an output side of the3D-LUT, and sample points are specified as the tops of hexahedrons(elliptical cubes) comprising quadrangles surrounding points at whichthe output values (RGB) are positioned in the color spaces. Commontetrahedral interpolation is then performed using these sample points todetermine the input values (RGB).

At step S1404, the PC 100 generates the combined LUT. More specifically,first, the PC 100 takes each piece of color component data calculated atstep S1403 as an input point. The PC 100 then acquires color componentdata (CMYK data) obtained by subjecting the corresponding colorcomponent data after color matching (color component data acquired atstep S1402) to color separation using the color separation table 408.Taking this color component data (CMYK data) as the output, the PC 100then associates therewith the corresponding respective pieces of colorcomponent data calculated at the step S1403 as input points to form thecombined LUT.

The PC 100 performs the above described processing of steps S1401 toS1404 for the matching tables 407 and the color separation tables 408corresponding to all the types of papers to be handled by the paper typeinput unit 410 of the correction processing unit 302 for a printer. ThePC 100 thereby generates a combined LUT that corresponds to all types ofpapers to be handled by the paper type input unit 410.

A conceptual diagram of the combined LUT that is obtained in this manneris shown in FIG. 8.

As described above, according to the present embodiment, the informationprocessing apparatus generates a combined LUT that performs colormatching and color separation in an integrated manner based on an LUTfor color matching and an LUT for color separation. The input points ofthe combined LUT are set such that the output that corresponds to theinput points of the combined LUT corresponds to input points of the LUTfor color separation.

Thus, when generating a single combined LUT by combining an LUT forcolor matching and an LUT for color separation, it is possible to reducedeterioration in the image quality of an image to be formed, whilesuppressing an increase in the data amount of the combined LUT.

Other Embodiment

The processing described in the above embodiments may be realized byproviding a storage medium, storing program codes of software realizingthe above-described functions, to a computer system or apparatus. Byreading the program codes stored in the storage medium with a computer(or a CPU or MPU) of the system or apparatus and executing them, thefunctions of the above-described embodiments can be realized. In thiscase, the program codes read from the storage medium realize thefunctions according to the embodiments, and the storage medium storingthe program codes constitutes the invention. The storage medium, such asa floppy® disk, a hard disk, an optical disk, a magneto-optical disk andthe like can be used for providing the program codes. Also, CD-ROM,CD-R, a magnetic tape, a non-volatile memory card, ROM, and the like canbe used.

Furthermore, the functions according to the above embodiments arerealized not only by executing the program codes read by the computer.The present invention also includes a case where an OS (operatingsystem) or the like working on the computer performs part or the entireprocesses in accordance with designations of the program codes andrealizes the functions according to the above embodiments.

Furthermore, the program codes read from the storage medium may bewritten in a function expansion card which is inserted into the computeror in a memory provided in a function expansion unit which is connectedto the computer. Thereafter, a CPU or the like contained in the functionexpansion card or unit may perform part or the entire processes inaccordance with designations of the program codes and may realize thefunctions of the above embodiments.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2006-344667, filed on Dec. 21, 2006, which is hereby incorporated byreference herein in its entirety.

1. A method of generating a combined look-up table for converting afirst color component data into a third color component data bycombining a first look-up table for converting by color matching thefirst color component data that is generated and input by a firstapparatus, and represented in a first color space into a second colorcomponent data that is represented in the first color space taking intoconsideration color reproduction characteristics of a second apparatus,and a second look-up table for converting by color separation the secondcolor component data into the third color component data that isrepresented in a second color space used by the second apparatus,comprising a step of generating a combined look-up table in which colorcomponent data after color separation that corresponds to each inputpoint of the second look-up table is made to correspond withcorresponding color component data before color matching in a case inwhich each piece of color component data corresponding to each inputpoint of the second look-up table is taken as color component dataobtained using the first look-up table.
 2. The method according to claim1, wherein the first color space is an RGB color space.
 3. The methodaccording to claim 1, wherein the second color space is a CMY colorspace or a CMYK color space.
 4. An image processing apparatus thatperforms color matching and color separation using a combined look-uptable that is generated by a method according to claim
 1. 5. An imageforming apparatus comprising an image processing apparatus according toclaim
 4. 6. A computer program stored in a computer-readable storagemedium for causing a computer to execute a method according to claim 1.7. A computer-readable storage medium on which a computer programaccording to claim 6 is recorded.